The invention relates to a projection screen, particularly a projection screen for large-surface image projection.
For large-surface image projection, as e.g. in movie theaters, use is made of projection screens. Images are projected onto the projection screens from a projector arranged on the side of the viewers. The light incident on the projection screen is scattered by the projection screen so that the image becomes visible to the viewer on the projection screen. Since such projection screens will scatter also extraneous light, such as e.g. daylight, the room has to be darkened. Outdoors use of such projection screens is possible only at night, while also in this case it must be precluded that extraneous light, e.g. from streetlamps, is allowed to impinge on the projection screen. In projection screens of this type, already minor fractions of extraneous light will cause a decrease of the image quality.
Further, projection screens are known wherein the projector is arranged on the rear side of the projection screen. In this case, the image is formed by back-projection on the projection screen which again is provided to scatter incident light.
In the periodical APPLIED OPTICS (12; 1973) 21.80-84, a microfilm display device is described wherein a combination of a diffusing screen and a lens is realized by means of a hologram. The hologram serves as an viewing screen for visualization of the microfilm images which are projected via a magnifying lens. The hologram, when illuminated by any light, appears as a ground glass, i.e. it is milky and opaque. The purpose of the hologram resides in obviating the need for a voluminous and expensive lens.
Known from DE 43 30 384 A1 is an illuminated panel having image holograms, forming surface or line patterns, stored therein. The holograms in the fields of various colors have been generated under different angles of illumination so that the fields will present different colors to the user. Only the stored pattern can be visualized in each given case.
It is an object of the invention to provide a projection screen which is useful particularly for large-surface image projection also if the fraction of extraneous light is large.
According to the invention, the above object is achieved by the features of claim 1.
The projection screen according to the invention comprises a plurality of holographic-optical elements arranged in one plane. On the holographic-optical element, forming a closed surface, an image can be generated by back-projection. In doing so, each individual holographic-optical element will present one image portion of a projected image. In this manner, the image to be projected is subdivided into individual pixels, as it were, and each pixel is represented by a holographic-optical element.
Each holographic-optical element comprises at least one diffraction grating and at least one lens. The diffraction grating is arranged to diffract light bundles of e.g. red, green and blue chromatic components of the to-be-projected image in a different manner. Through the lens, the image to be projected is represented in an image plane.
The holographic-optical elements used in the projection screen of the invention are provided in such a manner that the light bundles of different chromatic components of an image to be projected are respectively combined to form a common exiting light bundle with an identical light exit angle. Thus, each holographic-optical element will emit, in the direction of the viewer, a single light bundle containing the complete chromatic information of the respective image portion. Accordingly, each holographic-optical element generates one directional light bundle. The holographic-optical elements used in the inventive projection screen are transparent, and the light incident on the holographic-optical elements will undergo merely a small diffraction. Thus, also extraneous light is approximately not diffracted. Further, the contrast and the luminosity of the projection screen of the invention are superior to those in conventional projection screens. Therefore, the projection screen is also suited for use in large-surface image projection at daylight.
The projection screen, when viewed from the intended direction, will effect a perception of color images comprising a combination of the different color contents. When viewed from a different direction, however, the projection screen is clear and transparent. The projection screen is transparent also from the rear side. The projection screen can comprise e.g. the window pane of a room. With the projectors arranged inside the room and directed to the window pane, the projected image is visible from outside. Nonetheless, daylight is allowed to enter the room from outside, and persons within the room may look outside through the window without seeing the image.
Preferably, the light exit angles of holographic-optical elements spaced in the vertical direction differ from each other in a manner causing the light exit bundles to intersect each other in a viewing plane. Further, the holographic-optical elements are preferably configured to the effect that the exiting light bundles are widened only horizontally. Due to this horizontal widening, light of the exit bundles will enter the eye of the viewer also from holographic-optical elements arranged in the side region of the projection screen. Since the exiting light bundles are not widened in the vertical direction, losses of light intensity are avoided.
Preferably, two types of holographic-optical elements are used for the projection screen. The first type comprises a single diffraction grating and one or a plurality of lenses. For causing the diffraction grating to emit a common light bundle in the direction of the viewer, the rear side of the holographic-optical element is irradiated on by light bundles separated according to chromatic components. Thus, projected onto the holographic-optical element are a light bundle comprising the red chromatic components, a light bundle comprising the green chromatic components and a light bundle comprising the blue chromatic components. The angles between the light bundles are selected in such a manner that the light bundles, after undergoing their specific diffraction according to the chromatic components by use of the diffraction grating, will coincide to form a common exiting light bundle.
In the second type of holographic-optical elements, only a common light bundle, containing all of the chromatic components of the image to be projected, is projected onto the rear side of the holographic-optical element. The common light bundle thus includes all light bundles of the individual chromatic components. The holographic-optical element comprises an individual grating for each chromatic component, and again one or a plurality of lenses. The diffraction gratings are suitably adjusted to each other to the effect that the individual chromatic components of the incident light bundle will not be separated from each other but will be issued in the direction of the viewer as a common exiting light bundle with an identical light exit angle.
Further, the two above types of holographic-optical elements can be combined and correspondingly modified in that the fundamental colors magenta, cyan and yellow are used instead of the fundamental colors red, green and blue.
In the projection system according to the invention, use is made of a projection screen comprising the first type of holographic-optical elements. Further, the projection system comprises a projector for generating the individual light bundles of the chromatic components. Since the rear side of the holographic-optical elements of the first type is illuminated by light bundles separated according to chromatic components, the projector comprises one projection device for each light bundle to be generated. According to the invention, the distance of the projection devices is adjusted in such a manner to the color-specific diffraction angles of the diffraction grating included in each holographic-optical element that each holographic-optical element combines the light bundles of the chromatic components into a common exiting light bundle with an identical light exit angle. In this projection system, the projector to be used can be a RGB projector generating a red, a green and a blue chromatic component of the image to be projected. The projection system is adapted to generate images with high luminance and high contrast because the complete light projected by the projector onto the rear side of the projection screen is diffracted in the direction of the viewer.
In a further projection system according to the invention, use is made of a projection screen comprising holographic-optical element of the second type. The projector of this projection system includes only one projection device. The projection device is arranged to project a common light bundle, comprising the individual light bundles per chromatic component, onto the rear side of the projection screen. Each holographic-optical element comprises a respective diffraction grating for each light bundle of a chromatic component. The diffraction gratings are adjusted to each other to cause each holographic-optical element to emit a common light bundle in the direction of the viewer. Since, in this projection system, there is provided for each light bundle a separate grating in each holographic-optical element, the luminance in this projection system is lower than in the projection system described above. Still, the projection system can be utilized for large-surface image projection under daylight conditions. A particular advantage of this projection system resides in that the system allows the use of a conventional projector emitting white light.
In the two above projection systems, the projector is normally arranged not centrally relative to the projection screen but at a displacement in the vertical direction. The resultant distortion of the image projected onto the projection screen can be eliminated by arranging rectification lenses in the path of rays of the projection device or the plurality of projection devices. Further, the projector can have assigned thereto an electronic rectification device by which the image to be projected is suitably distorted to generate an undistorted image on the projection screen.